


A Crash Course in Monster Souls

by Toonpel



Category: Undertale (Video Game)
Genre: Brotherly Affection, Clueless Papyrus is Clueless, Curiosity, Humor, M/M, Poor Sans, Soul Exploration, Soul Sex, Unintentional Incest, or is he...?, puns, what even is this
Language: English
Status: Completed
Published: 2019-08-22
Updated: 2019-08-22
Packaged: 2020-09-24 05:13:52
Rating: Mature
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,902
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/20352955
Author URL: https://archiveofourown.org/users/Toonpel/pseuds/Toonpel
Summary: Monster Souls are pretty complex, and a trip to the Librarby confirms just how true that notion is.But hey, Sans is willing to do anything for Papyrus, even if that means getting subjected to his brother's innocent curiosity.One thing's for sure. If a reset ever happens again, Sans intends to make it his personal mission toneverlet Papyrus near that book again. Ever.





	A Crash Course in Monster Souls

**Author's Note:**

> Forgive me for this, I have no excuse.
> 
> Also, this is my first time writing any kind of sexual content, so, uhhh...idk.
> 
> Enjoy?

Sans and Papyrus stood before a large collection of books, every single one of them sorted and arranged by color. If you were a bookworm, it would be easy to get swept away into a world of written glory, from facts to fiction and everything in between.

Sans swept his phalanges over the spines of the various tomes, scanning their titles with only half a mind. He glanced over at his brother, who was by no means being gentle to any of the books he deemed interesting, letting out small _nyeh's_ each time he found something he liked.

Sans smiled with undisguised affection, his eyes shining with adoration. The sight of that sunny disposition and radiating warmth proved to be the most interesting thing around. Heck, the books here might as well just pack up, _book_ it and turn a new page. Heh.

Papyrus was the best.

Though, as it turned out, the skeleton brothers' most recent visit to the Librarby would soon prove to be a mistake, although Sans did not know that yet. The visit itself had been stirred by a spark of curiosity from Papyrus, the tall skeleton proclaiming that his great collection of books should naturally grow in size to match his own growing greatness. That, and he had gotten bored of the same bedtime stories being read to him each night - Fluffy Bunny included. It might have been Papyrus' favorite, but even that one had to _hop_ over and take a backseat at times.

That was why his brother had hauled the two of them to the Librarby by hand, making Sans promise his little bro he would read whatever story he fancied. The short skeleton couldn't help spoiling Papyrus, he was just too damn precious.

"SANS, I HAVE RETRIEVED THE PERFECT BEDTIME STORY FOR TONIGHT'S OCCASION! NYEH HEH HEH!"

Case in point. Sans snickered along with his bro's animated laugh, walking up to get a sneak peek of tonight's lucky winner.

"that's great pap, what did you find?"

"WHY, ONLY THE BEST, OF COURSE!" Papyrus exclaimed, but then paused briefly to look down at the book's cover. Sans, much to his own inner frustration, was far too short to see it himself when it was being held by his tall sibling.

"ACTUALLY, I HAVE NO IDEA WHAT IT'S ABOUT, BUT IT SOUNDS POSITIVELY ENTHRALLING!"

Sans stood on the tips of his toes, the struggle of being small far too real as his curiosity grew by the second. He eventually gave up when Pap started looking through the book, oblivious to Sans' growing frustration.

"hey, uh, bro." The older brother eventually said. "can i see the book? i'm a little _short_ on its content."

"OH!" Papyrus looked down at him, ready to give him the book when he suddenly frowned, the pun registering. "SANS, THAT ONE WAS JUST BAD!"

Sans' grin widened. "eh, what can i say. the rest of my puns just _booked_ it. they couldn't handle being _worded_." He winked.

Papyrus groaned. "OH MY GOD, JUST TAKE A LOOK, YOU BONEHEAD!" He said, handing the book over. Sans took it with a chuckle, gazing at the cover.

And then promptly choked on his own laugh, almost dropping it.

'_The Science and Biology of Monster Souls_', it read.

Sans felt like this book had just slapped him.

"wh-" He began, blinking. "where did you find this, pap?"

"NYEH HEH, IMPRESSED, BROTHER!? ONLY I COULD FIND SUCH A DEVIOUSLY HIDDEN BOOK! IT WAS A SIMPLE MATTER OF LOOKING BEHIND THE OTHER BLUE BOOKS!"

"oh. ok, gotcha." He said, patting Papyrus' lower arm. "yeah, you're just that great, bro."

The tall skeleton puffed out his chest, beaming. "I'M GLAD YOU NOTICED!"

He then proceeded to pick Sans up, the small skeleton immediately latching on with his free hand.

"COME, BROTHER! A NEW BEDTIME STORY AWAITS!"

As Papyrus hauled them both out of the Librarby, Sans rested his skull against Pap's chest, glaring at the book in his free hand. He didn't have to be perceptive to know this spelled disaster.

Sans continued to stare at the book while Papyrus got ready for bed, mentally preparing himself for what was about to come. Hopefully, Paps wouldn't be too curious after hearing some of the things it had to offer, but Sans didn't count on his lucky stars. Papyrus was a curious soul, among many other things.

"THE GREAT PAPYRUS IS READY FOR AN EQUALLY GREAT BEDTIME STORY, BROTHER!"

That was his cue.

Sans plopped down on the chair he had dragged along with him and opened the book. He gave Papyrus a brief glance, taking note of his huge grin and sparkling eyes. There was no going back, was there?

Scratching the back of his skull, Sans began to read.

"a monster's body is attuned to their soul, making it a vital part of their being. it is by that definition also considered to be everything that makes up a monster, defining it from appearance to personality and likewise the magic they carry."

Okay, so far so good. This book didn't seem _too_ bad, only stating facts rather than...something else.

He glanced up at Papyrus, who looked no more tired than he had before the story began...which was to say he didn't look tired at all.

Sans resumed his reading.

"studies show that a monster's soul is made up of love, hope and compassion, but the true nature of a soul has yet to be confirmed. after all, humans have proven they don't need these traits to exist."

Jeez, that took a quick turn.

"while it is widely known that a monster's soul is weaker than that of a human's, it has been discovered that the former uses their soul for far more than a human ever could. for instance, the nature of soul mates and soul bonding..."

Hoo boy, here we go. Sans began to sweat as he stared at the page for a few agonizing seconds, his smile twitching.

"PLEASE CONTINUE, DEAREST BROTHER!" Papyrus said, breaking the awkward silence that had formed.

Sans blinked. Right.

Clearing his non-existent throat, the short skeleton went over the pages to the best of his ability, cringing inwardly whenever something particularly..._heated_ was mentioned. This really wasn't the kind of thing he wanted his sweet, innocent little brother to hear.

When he eventually reached the sexual parts of the book, Sans promptly closed it.

"nope." He said.

Papyrus whined. "SANS, WHY DID YOU STOP? WE WERE JUST GETTING TO THE GOOD PART!"

Sans' left eye socket twitched. "n-nah, bro." He said, placing the book on the ground. "the rest of it was nothing more than scientific research and equations. ya know, all that boring stuff only nerds like to read about." He lied through his teeth.

Papyrus turned his head on the pillow, looking at him.

"OH...WELL THAT'S OKAY THEN!" He smiled, and Sans couldn't help but sigh in relief. "BUT WOWIE, THAT SURE WAS MIND-BLOWING! I NEVER KNEW OUR SOULS WERE SO AMAZING...AND COMPLICATED!"

Sans chuckled. "heh, yeah. you could say that's our _soul_ defining trait."

"UGH, SANS, IT IS FAR TOO LATE FOR YOUR SILLY PUNS!" His bro scolded, but the short skeleton didn't miss the smile on his face.

Sans was about to reply when Papyrus continued, a curious expression replacing his smile.

"BUT REALLY, SANS. I AM QUITE CURIOUS. I CERTAINLY CAN'T REMEMBER EVER WITNESSING ANOTHER MONSTER'S SOUL, ASIDE FROM MY OWN OF COURSE." He mused, his eyes squinting as he thought hard. "DO THEY ALL LOOK THE SAME?"He turned to Sans. "HOW DOES YOURS LOOK, BROTHER???"

Oh no. Don't panic, don't panic, don't--

"uhhh, i dunno."

Sans was panicking.

"GASP!!" Papyrus, well, gasped. "HOW CAN YOU NOT KNOW WHAT YOUR SOUL LOOKS LIKE, BROTHER!?!?"

"no, well i-"

"WE MUST RECTIFY THIS AT ONCE!" Papyrus interrupted. "NOW BRING IT FORTH, SANS!"

By now the short skeleton was pretty much sweating bullets.

"what. no way, i-" He paused, taking in the open concern his sibling was exuding. Had he actually managed to worry Papyrus with his idiotic response? Sans had seen his own soul plenty of times, for crying out loud.

He could just tell the truth, but...Sans didn't want to upset Papyrus now that he'd gotten the absurd idea he needed help with his soul.

He resigned himself to his fate.

"ok." Sans said, bringing his left hand up to his chest. "ok. just be...careful, alright? monster souls are frail, remember?"

"OF COURSE, BROTHER! THE BOOK EXPLAINED THAT QUITE WELL, AND I AIM TO BE NOTHING BUT CAREFUL!"

"i know paps. i trust ya."

And he did trust Papyrus, which was why Sans was letting this happen in the first place. Taking a deep breath, the older brother drew forth his soul, the small, inverted little heart shining a dull white light that pulsed weakly with his entire being.

Sans felt naked the moment it left his ribcage.

Papyrus' eye sockets widened. "OH, WOWIE, YOURS DOES LOOK EXACTLY LIKE MINE, SANS!" He paused briefly, studying the soul before him. "WELL, ALMOST LIKE MINE. YOURS IS WAY SMALLER AND NOT AS SHINY, BUT THAT MAKES PERFECT SENSE, NYEH HEH HEH!"

Sans forced out a chuckle, feeling self-conscious. Papyrus patted his skull, making him turn his eye lights on him. There was a gentle smile resting on his brother's face.

"YOUR SOUL IS BEAUTIFUL, SANS."

Heat rose to his cheeks unabated, a fluttering warmth coursing through his bones. Sans averted his gaze, not really sure how to take such a compliment addressed to his entire being.

"heh, thanks pap." He said earnestly. "but i bet yours is even more be- _gah_-!" Sans yelped mid sentence, a small jolt of pleasure shocking him.

He whipped his head back to look at Papyrus, who had grabbed his soul and _poked_ it.

Sans breathed deeply as he forced himself to calm down. It was okay, this was going to be okay. His brother was only curious - that was part of his nature, after all. He would let go soon, Sans was sure of that.

Instead, Papyrus only continued, poking and fondling the little heart. "NYEH, YOUR SOUL IS REALLY SOFT AND SQUISHY, BROTHER! DID YOU KNOW THAT?"

"ah...w-well."

Sans bit back a startled gasp as one pleasurable jolt after another assaulted him, heating his bones and making his breath ragged. He twitched and jolted with each touch, finding it increasingly hard to keep from making inappropriate little noises.

God, was this really happening? What kind of nightmare had Sans found himself in?

His brother then proceeded to caress it like it was the most precious thing in the world, making Sans shudder violently.

"nh...hhh"

The petting stopped.

"OH MY GOD, SANS!?"

The short skeleton looked up at Papyrus, his eye lights hazy as sweat trickled down his skull. His brother looked absolutely mortified, unconsciously squeezing his soul even further.

"nn...p-pap, please..." He breathed, clutching at his white shirt. "l-let go bro..."

"I'M SO SORRY, BROTHER! ARE YOU OKAY!? DID I HURT YOU!?" He whimpered as he let go of the inverted heart, worry written all over his face.

Sans managed to calm down enough to breathe evenly, wiping sweat off of his heated face.

"n-nah, bro. don't worry, you didn't hurt me at all." He said, closing one eye socket. It wasn't even a lie, not really. "you just startled me, heh."

"THANK GOODNESS, I DON'T KNOW WHAT I WOULD HAVE DONE IF I ACCIDENTALLY BROKE YOU!!"

Papyrus leaned forward to scoop Sans up from the chair, pulling him back with him to rest on his lap. Sans closed his eye sockets and just focused on staying calm, the sudden heat tingling his bones making him feel nauseous.

"it'll take a _lot_ more than your affectionate pats to break me, pap."

"NYEH!" Papyrus said, hugging him tighter. Eventually, the tall skeleton loosened his grip, turning his attention back to the floating heart.

He then gave it another pat. Sans' breath hitched.

"DID THAT HURT!?"

Sans sighed, feeling extremely tired. "...nah."

Papyrus looked curious again. "OH...SO, DOES IT FEEL GOOD THEN!?"

Sans placed his skull against Papyrus' chest, opening his eye sockets. "in a sense, i guess."

What an understatement. He just wanted this to be done and over with, returning his soul back to where it belonged without having to worry about any sudden _arousals_ brought forth by his _brother_, of all people.

"WELL THEN...!" The tall skeleton began, looking nervous. "CAN I GIVE YOU MORE BROTHERLY PATS? IF IT MAKES YOU FEEL GOOD, THEN I'D LOVE TO GIVE YOU ALL THE AFFECTION YOU DESERVE!"

No. No, just say no. It was one simple word, just a single syllable...and yet Sans couldn't for the _life_ of him get it out. It felt like he was having an internal battle, one side desperately wanting this to stop, and yet another side wishing for nothing more than to please his brother...maybe Sans _was_ spoiling Papyrus a bit too much.

Just say no, get it over with.

"...sure."

Sans felt something within him shrivel up and die.

Papyrus began caressing his soul with a gentle touch, and Sans could tell he was being extra careful now. Still, that did little to the heat building within him, his cheekbones feeling like they were literally on fire. He clutched his brother's pajama shirt, trembling slightly as his breathing became uneven once again.

"hhh...mnh...m.."

Papyrus brought his other hand up to the little heart, squeezing it lightly between his thumb and pointy finger. Sans' hips twitched as a response, and an immense humiliation crashed over him - along with another spark of pleasure.

His brother then began fondling it again, squeezing it every once in a while.

"ng...hh.." Sans' phalanges tightened around the fabric of Papyrus' shirt, trying to stay as silent and motionless as possible, but the overwhelming care and attention Paps gave his soul made it almost impossible. After yet another pleasurable jolt, Sans involuntarily jerked his hips forward, his pelvis hitting nothing but air due to the position he was in. This in turn made Sans feel both extremely relieved and frustrated.

It was all becoming a bit too much. He squeezed his eye sockets shut as he felt his soul begin to leak, dripping with the pleasure that was given to him (albeit unintentional on both parts). Sans shuddered, his hands twitching with a sudden urge to touch his heated bones.

The fondling stopped, and the small skeleton felt his brother's touch leave his soul.

"SANS, WHAT'S HAPPENING NOW!? YOU'RE LEAKING!"

Sans almost whined. He didn't want this, he _really_ didn't, but Papyrus couldn't just stop now - not when he was feeling this much sexual frustration. And all because of that _stupid_ book.

"hh...eh, d-don't worry bro, that's, uh...normal...?"

"OH..." Papyrus said. "ARE YOU SURE???" He looked down at Sans, taking in the complete mess that he was.

"y-yeah...?"

Papyrus still looked uncertain. "WELL...IF YOU'RE CERTAIN."

When he felt his soul being touched again, Sans nearly let out a moan from the sheer sensation. At this rate, he wasn't going to last long. God, this was so, _so_ wrong.

Once his pleasure reached its peak, Sans let out a shuddering breath and his entire body froze up, his soul now dripping with his climax.

"hhh...hmff...hh.." Sans panted, his bones coated in sweat. He released his death grip on Papyrus' shirt, curling into a ball as he finally, _finally_ let his soul drift back into his body. He stayed like that for a while, enjoying the silence that followed.

"HMM..." Papyrus eventually said, dispelling the quiet. "I'M...NOT SURE THIS WAS SUPPOSED TO HAPPEN?" He looked down at the small skeleton in his lap. "SANS, WHAT JUST HAPPENED?"

"hh...w-well, you, uh...you gave me a-a good time...?"

"SO YOU FEEL OKAY? DID I GIVE YOU ENOUGH BROTHERLY AFFECTION?"

"pfft, h-heh heh...w-way more than needed, paps."

_Way more_.

Papyrus actually looked proud. "WELL THEN, MISSION ACCOMPLISHED, NYEH HEH HEH!"

He grabbed Sans again and placed him in his arms, hoisting them both up from his race car bed. "WELL, YOU CERTAINLY LOOK EXHAUSTED, SO I'LL CARRY YOU TO YOUR ROOM THIS TIME! AFTER ALL, I AM A GREAT BROTHER!"

Sans chuckled weakly. "...yeah. you're the best."

"EXACTLY!"

Once he was placed down on his old mattress, Papyrus patted his skull a few times and stood back up.

"GOOD NIGHT, BROTHER, AND THANK YOU FOR THE BEDTIME STORY!"

The small skeleton curled into a ball again. "no probs, pap."

"SLEEP WELL SANS, TOMORROW IS ANOTHER DAY A HUMAN MIGHT SHOW UP! WE MUST BE READY, AS ALWAYS!"

"of course bro, i wouldn't have it any other way."

"AND...!" There was a brief pause, before Papyrus spoke up again, voice soft. "I LOVE YOU, BROTHER."

Sans closed his eye sockets. "i love ya too, papyrus."

Once the tall skeleton left his room, Sans promptly fell asleep. When he awoke again later that night, the short skeleton briefly played with a thought.

Sans would, without a single doubt, find the one moronic enough to place such a book in an _open library_, where everyone (aka Papyrus) could find it. And, needless to say, whoever he would find was in for a very b a d t i m e.

...

Somewhere in Hotland, a small yellow lizard shivered.

**Author's Note:**

> So...is Papyrus actually as innocent and clueless as Sans seems to think?
> 
> I'll leave that up to you ;)


End file.
